1. Field of the Invention
The present invention relates to a method and to apparatus for controlling the transmission ratio of a continuously variable transmission for a motor vehicle.
2. Description of the Related Art
One form of a continuously variable transmission is described in German Patent Publication No. DE 19 546 294 A1. A continuously variable transmission generally includes pairs of conical sheaves in the form of a pair of input sheaves and a pair of output sheaves. Each pair of sheaves includes an axially displaceable sheave and an axially stationary sheave to allow adjustment of the transmission ratio. Between the two pairs of sheaves is an endless, torque-transmitting member in the form of a drive belt for transmitting torque
During the time of operation of such a transmission, changes caused by wear occur, or changes that, for example, are attributable to the transmission itself. Other changes generally occur in transmissions once test-stand-derived parameter settings are changed. As a result, and depending on the duration of operation of a transmission, on wear condition circumstances, and the like, different demands are imposed, and those conditions can adversely influence the regulation accuracy and the operational behavior of the transmission.
An object of the present invention is to provide a method and an apparatus for regulating the transmission ratio of a continuously variable transmission, especially such a transmission intended for use in a motor vehicle, and to provide high operational performance of the transmission over long operating times.
Briefly stated, in accordance with one aspect of the present invention, a method for regulating the transmission ratio of a continuously variable transmission is provided. The method includes changing a set point value for the transmission ratio of the transmission as a function of operating parameters of a power train. The set point value is composed of a preliminary control value and a correction value, and the correction value is derived by comparing a measured transmission ratio with the set point transmission ratio. The preliminary control value is a function of at least two transmission operating parameters, and the relationship between the transmission operating parameters and the transmission""s set point value correction value is updated based upon changes that occur during transmission operation.
In accordance with another aspect of the present invention, apparatus is provided for regulating the transmission ratio of a continuously variable transmission. The apparatus includes sensors for detecting operational transmission parameters, and an electronic control unit including a microprocessor and at least one memory in which operating parameters associated with reference transmission ratios are stored. The electronic control unit further includes a preliminary control device in which a preliminary control value is determined as a function of at least one of the transmission operating parameters, and a regulator is provided for comparing a measured transmission ratio of the transmission with a reference transmission ratio and for deriving therefrom a control value. An adjusting device receives the preliminary control value and the regulation value as the set point value in order to adjust the transmission. The preliminary control value is stored in the preliminary control device as a function of at least two transmission operating parameters, and an adjustment system is provided which changes the preliminary control value so that the measured transmission ratio coincides with the reference transmission ratio when the control variable is at least approximately zero.